1. Field of the Invention
The present invention relates to a light source device using a white LED (light emitting diode).
2. Description of the Related Art
A halogen lamp has been used as, for example, a light source in a device that uses light to perform various measurements. For example, the halogen lamp has been used as the light source in a device that evaluates the performance of a lens, such as an MTF (modulation transfer function) measuring device. JP-A-2004-93322 discloses a structure in which a halogen lamp is used as the light source to analyze and measure the function of a sample. In addition, JP-A-2000-133005 discloses a structure in which a halogen lamp is used as the light source of a solar simulator.
In recent years, a white LED has been developed. Since the white LED has lower costs and lower power consumption compared to the halogen lamp, it is considered that the white LED can be used as a light source instead of the halogen lamp. A first type of white LED is obtained by combining a red LED, a green LED, and a blue LED. This type of white LED mixes color light components emitted from the LEDs and emits white light. A second type of white LED includes a blue LED chip that emits blue light and a fluorescent material that is excited by the blue light emitted from the blue LED chip and emits yellow light, and mixes the blue light emitted from the blue LED chip and the yellow light emitted from the fluorescent material to emit white light. However, the spectral characteristics (spectral distribution) of the white LED are different from those of the halogen lamp. Therefore, particularly, it is difficult to merely replace the light sources of various types of measuring devices with the white LEDs. For example, when the white LED is used as the light source in a device for evaluating the performance of a lens, the difference in spectral characteristics between the light sources causes the chromatic aberration of the lens.
FIG. 6 is a diagram illustrating the spectral characteristics of a typical halogen lamp. FIG. 7 is a diagram illustrating the spectral characteristics of a typical white LED. In addition, FIG. 7 shows the spectral characteristics of the second type of white LED. FIG. 8 is a diagram illustrating the spectral characteristics of the halogen lamp and the spectral characteristics of the white LED at the same time for comparison therebetween. In FIG. 8, a solid line indicates the spectral characteristics of the halogen lamp and a broken line indicates the spectral characteristics of the white LED. In FIGS. 6 to 8, the horizontal axis indicates a wavelength (nm) and the vertical axis indicates the quantity of light. In addition, the vertical axis indicates the quantity of light in an arbitrary unit.
As shown in FIG. 6, the halogen lamp has a spectral distribution in which an emission peak is in the intermediate wavelength range (yellow range) of visible light and the quantity of light is gradually reduced toward the short wavelength range (blue range) and the long wavelength range (red range) of the visible light. In contrast, the second type of white LED mixes the blue light emitted from the blue LED chip and light emitted from a yellowish fluorescent material by the blue light, which is excitation light. Therefore, as shown in FIG. 7, the second type of white LED has emission peaks in both the blue range and the yellow region. As shown in FIG. 8, the emission peak in the blue range is the main difference in spectral distribution between the halogen lamp and the white LED. Therefore, for example, when the halogen lamp, which is the light source in a device for evaluating the performance of a lens, is merely replaced with the white LED, a pronounced aberration of blue light is observed compared to when the halogen lamp is used as the light source.